1. Field of the Invention
The present invention relates to a method for preparing all-silica crystalline zeolites having the IFR structure (“IFR zeolite”).
2. State of the Art
Zeolites are an important class of microporous, crystalline solids which are used industrially in heterogeneous catalysis, adsorption, separation and ion-exchange. The properties of zeolites for these applications are strongly affected by the structural features such as the framework composition and topology. Benefiting from the hydrophobic properties of their electroneutral framework, pure-silica zeolites provide tremendous opportunities of separating nonpolar from polar molecules. As discussed below, it is often a challenge to synthesize such pure-silica zeolites. Therefore, it is desirable to find an economic and efficient way to reach this goal.
The direct synthesis is the primary route of the synthesis of zeolites. The major variables that have a predominant influence on the zeolite structure crystallized include the composition of synthesis mixture, temperature and time. Depending on the nature of the zeolites involved and the chemistry of their formation, some zeolite structures can be synthesized in a broad spectrum of framework compositions, as exemplified by ZSM-5 containing no heteroatoms (Si-ZSM-5, i.e., pure-silica ZSM-5), as well as ZSM-5 containing heteroatoms in its crystal framework (for example boron (B-ZSM-5, i.e., borosilicate ZSM-5), gallium (Ga-ZSM-5, i.e., gallosilicate ZSM-5) or aluminum (Al-ZSM-5, i.e., aluminosilicate ZSM-5)). By contrast, the synthesis of some other structures succeeds only if certain heteroatom X (X=B, Ga or Al, for example) is present in the synthesis mixture and, in turn, incorporated into the framework. In many cases, certain zeolite structures can be synthesized only with certain specific heteroatoms within a limited range of Si/X ratio or in the presence of certain specific structure directing agents (SDAs). These complicated relationships between zeolite structures, framework compositions and SDAs have been discussed in many publications and patents (see S. I. Zones et al. J. Am. Chem. Soc. 2000, 122, 263–273; U.S. Pat. No. 4,963,337, issued Oct. 16, 1990 to Zones et al.; U.S. Pat. No. 4,910,006, issued Mar. 20, 1990 to Zones et al.; and R. F. Lobo, M. Pan, I. Y. Chan, R. C. Medrud, S. I. Zones, P. A. Crozier and M. E. Davis, J. Phys. Chem. 1994, 98, 12040–12052.)
In addition to the direct synthesis method, post-synthetic treatments often provide an alternative route to modify the zeolites to acquire desirable framework compositions. The post-synthetic treatment techniques all operate on the same principle: the desirable atoms such as Al and Si are inserted into lattice sites previously occupied by other T-atoms such as B. For example, Jones et al. developed a method of making pure-silica zeolites post-synthetically via treatment of borosilicate zeolites with, for example, acetic acid to expel boron from zeolites and subsequently heal the defects created by deboronation with silicon dissolved from other parts of the crystal (see C. W. Jones, S. J. Hwang, T. Okubo, M. E. Davis Chem. Mater. 2001, 13, 1041–1050). Although some pure-silica zeolites can be prepared via such post-synthetic techniques, it is always desirable to have a more economic and efficient way to reach the goal via direct hydrothermal synthesis.
There are two hydrothermal routes for the synthesis of pure-silica zeolites: (1) using OH anion as a mineralizer at high pH, and (2) using F− anion as a mineralizer at near neutral pH. Synthesis of many pure-silica zeolites has succeeded so far only using the fluoride method. The drawbacks of the fluoride route are that fluoride ions (from HF or NH4F, for example) are involved in the synthesis and the crystallization usually takes a longer time than via the OH− route.
Zeolite SSZ-42, a known zeolite, is characterized by an undulating, one-dimensional 12-membered ring channel system (see C. Y. Chen, L. W. Finger, R. C. Medrud, P. A. Crozier, I. Y. Chan, T. V. Harris, S. I. Zones, J. Chem. Soc., Chem. Comm., 1997, 1775–1776; C. Y. Chen, L. W. Finger, R. C. Medrud, C. L. Kibby, P. A. Crozier, I. Y. Chan, T. V. Harris, L. W. Beck, S. I. Zones, Chemistry—A European. J. 1998, 4, 1312–1323; P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, Chem. Mater. 1997, 9, 1713–1715; and P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, J. Phys. Chem. B 1997, 102, 4147–4155.). According to the International Zeolite Association (IZA), the zeolites designated SSZ-42, MCM-58 and ITQ-4 have the same framework topology which the IZA has assigned the code “IFR” (see Ch. Baerlocher, W. M. Meier and D. H. Olson and, Atlas of Zeolite Framework Types 2001, Elsevier, p. 302).
Zeolites having the IFR framework topology can be synthesized in both borosilicate (B-IFR) and aluminosilicate (Al-IFR) form by using a variety of benzyl derivatives such as N-benzyl-1,4-diazabicyclo [2.2.2]octane, N-benzylquinuclidinium, N-benzylquinuclidinol or benzyltropanium cations as structure directing agents (“SDA”s) (see C. Y. Chen, L. W. Finger, R. C. Medrud, P. A. Crozier, I. Y. Chan, T. V. Harris, S. I. Zones, J. Chem. Soc., Chem. Comm., 1997, 1775–1776; COY. Chen, L. W. Finger, R. C. Medrud, C. L. Kibby, P. A. Crozier, I. Y. Chan, T. V. Harris, L. W. Beck, S. I. Zones Chemistry—A European. J. 1998, 4, 1312–1323; U.S. Pat. No. 5,653,956, issued Aug. 5, 1997 to Zones et al.; U.S. Pat. No. 5,441,721, issued Aug. 15, 1995 to Valyocsik; U.S. Pat. No. 5,437,855, issued Aug. 1, 1995 to Valyocsik; and C. Y. Chen, S. I. Zones, L. T. Yuen, T. V. Harris and S. A. Elomari, Proc. 12th Int. Zeolite Conf. 1998, 1945–1952.).
U.S. Pat. No. 5,653,956, issued Aug. 5, 1997 to Zones, discloses zeolite SSZ-42 (a zeolite having the IFR framework topology), methods of preparing it and its use in, e.g., catalysts for hydrocarbon conversion reactions. The SSZ-42 is prepared using a N-benzyl-1,4-diazabicyclo[2.2.2]octane cation or N-benzyl-1-azabicyclo[2.2.2]octane cation.
U.S. Pat. No. 5,653,956 does not, however, disclose all-silica SSZ-42 or the method for its preparation. For example, the SSZ-42 zeolite is said to have a mole ratio of an oxide selected from silicon oxide, germanium oxide and mixtures thereof to an oxide selected from aluminum oxide, gallium oxide, iron oxide, titanium oxide, boron oxide and mixtures thereof greater than 10 (see col. 2, 1. 54–61). Likewise, as-synthesized SSZ-42 is described as having a YO2/W2O3 mole ratio of greater than or equal to 15, where Y is selected from the group consisting of silicon, germanium and mixtures thereof and W is selected from boron, aluminum, gallium, iron, titanium and mixtures thereof wherein at least 50% of W is boron (see col. 3, 1, 1–15). Thus, it appears that at least one oxide other than silicon oxide is present in the as-synthesized SSZ-42. This is confirmed in Examples 4 and 16 (the examples in which SSZ-42 is synthesized directly) where the product contains silicon along with boron or a mixture of boron and aluminum.
It is further disclosed in U.S. Pat. No. 5,653,956 that some or all of the boron in SSZ-42 may be replaced with at least one other element (see col. 8, 1.11–21 and Examples 14–16). However, there is no suggestion as to how an all-silica SSZ-42 can be made, and, in any case, the boron replacement is a post-synthesis step and not part of the direct synthesis of SSZ-42.
U.S. Pat. No. 5,437,855, issued Aug. 1, 1995 to Valyocsik, discloses zeolite MCM-58 (which, according to the IZA has the IFR framework topology), a method for its preparation and its use in the catalytic conversion of organic compounds. It is disclosed that the MCM-58 is prepared using a benzylquinuclidinium cation SDA. The MCM-58 has a composition containing an oxide of a tetravalent element (such as silicon, tin and/or germanium) and an oxide of a trivalent element (such as aluminum, boron, iron, indium and/or gallium). See col. 2, 1. 38–60, col. 4, 1. 40–55 and Examples 1–11. U.S. Pat. No. 5,437,855 does not, however, disclose all-silica MCM-58 (or any other all-silica zeolite having the IFR framework topology) or a method of making all-silica MCM-58.
U.S. Pat. No. 5,441,721, issued Aug. 15, 1995 to Valyocsik, also discloses zeolite MCM-58 (which, according to the IZA has the IFR framework topology), a method for its preparation and its use in the catalytic conversion of organic compounds. In this case, however, it is disclosed that the MCM-58 is prepared using a benzyltropanium cation SDA.
Like U.S. Pat. No. 5,437,855, the MCM-58 of U.S. Pat. No. 5,441,721 has a composition containing an oxide of a tetravalent element (such as silicon, tin and/or germanium) and an oxide of a trivalent element (such as aluminum, boron, iron, indium and/or gallium). See col. 2, 1. 33–56, col. 4, 1. 34–55 and Examples 1–6. U.S. Pat. No. 5,441,721 does not, however, disclose all-silica MCM-58 (or any other all-silica zeolite having the IFR framework topology) or a method of making all-silica MCM-58.
ITQ-4, which is a pure-silica zeolite having the IFR framework topology (see P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, Chem. Mater. 1997, 9, 1713–1715; P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, J. Phys. Chem. B 1997, 102, 4147–4155; M. A. Camblor, A. Corma, L. A. Villaescusa, J. Chem. Soc., Chem. Commun. 1997, 749–750; and P. A. Barrett, E. T. Boix, M. A. Camblor, A. Corma, M. J. Diaz-Cabañas, S. Valencia, L. A. Villaescusa, Proc. 12th Intern. Zeolite Conf. 1998, 1495–1502 and WO9829332, published Jul. 9, 1998.) is one of the examples among pure-silica zeolites synthesized by Camblor et al. via the fluoride route. Camblor et al. found that fluoride ions reside within the small [43526] cage located around the periphery of the central pore space of ITQ-4, showing some “templating” role for the formation of this structure (see P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, J. Phys. Chem. B 1997, 102, 4147–4155.). Camblor et al. pointed out that “the key parameter for the synthesis of ITQ-4 is the presence of F− anions, with a wide tolerance existing toward changes in pH.” (see P. A. Barrett, M. A. Camblor, A. Corma, R. H. Jones, L. A. Villaescusa, Chem. Mater. 1997, 9, 1713–1715.).
The present invention provides, in contrast to the synthesis method of Camblor et al., a new method for the direct synthesis of all-silica zeolites having the IFR framework topology via the OH− route without using F− anions. These results are important since the synthesis medium consists only of a silicon source, SDA solution and, an active source of hydroxide (such as alkali metal hydroxide, alkaline earth metal hydroxide, ammonium hydroxide and/or the SDA in its hydroxide form). This result is also important because, aside from corrosion issues of the Camblor et al. F− synthesis, the method provides about double the yield of product per mole of silica compared to the F− method.